1. Field of the Invention
The present invention relates to semiconductor devices, and more specifically to a method and apparatus for enhanced contact opening and via lithography.
2. Background Information
In the semiconductor industry, there is a continuing trend toward higher density devices. To achieve these high densities, there has been and continues to be efforts toward scaling down device dimensions. To this end, numerous lithographic methods and apparatus have been investigated to achieve acceptable performance in the deep sub-micron regime.
Many different approaches may be used to increase the resolution of the lithographic process. As used herein, resolution may refer to the dimension of the smallest resolvable feature patterned by a projection system in a photosensitive layer. For example, since the resolution of the optical projection system is proportional to .lambda./NA, where .lambda. is the wavelength of the exposing radiation, and NA is the numerical aperture of the projection system lens, the dimension of smallest resolvable feature patterned by such projection system may be decreased by decreasing the wavelength of the exposing radiation and/or increasing the numerical aperture of the projection system. While there has been some success with this approach, there are several drawbacks. For example, since the depth of focus is proportional to .lambda./NA.sup.2, decreasing the wavelength, and, to a greater extent increasing the numerical aperture, decreases the depth of focus which results in degradation of one's ability to control minimum dimension features in the allowed range of tolerances over the given range of underlying feature topologies. Additionally, other problems exist, such as the increased expense and complexity of systems with the large NA, and the long exposure times, reticle technology, overlay problems and significant other challenges yet to be overcome for very short wavelengths (e.g. E-Beam, X-ray, Ion beam, etc.) technology.
Of particular concern is the patterning of openings such as contact openings and vias in the photo-resist layer. In general, a contact opening is a small dimension square or circular opening in an insulative layer which exposes active regions of a transistor (e.g. source, drain and gate for MOS transistor, emitter, base and collector for bipolar transistor or BJT) to allow for electrical connection thereto, while a via is a small dimension opening in an insulative layer to an underlying metallization layer to allow for electrical connection thereto. Typically, contact/via lithography is less robust than, for example, minimum dimension line and/or space lithography from the same generation of technology due to the poor ability to control variation in the opening size. Primarily, this inability to control the opening size is caused by a relatively faster loss of feature dimension control in out-of-focus conditions for small, evenly spaced openings as compared to lines and/or spaces. In addition, small (close to the resolution limit) openings are subject to significant deviation in their shape in the presence of even small aberrations in the projection system. Further, the limited ability to see the "bottom" of the patterned image of the contact in, for example, SEM microphotographs, usually compounds the problem by limiting the ability to obtain meaningful results of the contact/via lithography process. Moreover, a variance in the feature size on the reticle results in a greater impact on size variation of the patterned features for contacts and vias compared with lines and/or spaces for features with sizes close to the minimum.
The minimum resolution of manufacturing worthy contact/via process (process where degree of feature size control is adequate to meet design tolerances) may be expressed as A.lambda./NA, where A is a constant dependent upon the particularly adopted lithographic process. Typically, the value of A is in the range of approximately 0.7-1.0 for many contact/via lithography processes. In contrast, the resolution of line and/or space lithography is typically approximately 0.6/.lambda./NA or smaller for advanced manufacturing processes. It is worth noting that the value of A can be reduced significantly if the minimum feature patterned is situated on a pitch that is at least 3 times as large as minimum feature size (or fully isolated), so patterning is less contrast constrained than the case when minimum feature is considered in the context of the pitch that is twice as large (or less) as minimum feature size.
Referring to FIGS. 1 and 2, an illustration of the contact/via control problem is shown. In FIGS. 1-2, size variability of a patterned feature due to reticle error is illustrated. In these illustrations, other causes of size variability, such as lens aberrations, are not present. In the graph of FIG. 1, an aerial image of a periodic space is shown. The aerial image is produced for a positive tone reticle having a periodic space with a dimension of 300 nm +/-12 nm at the wafer plane. That is, the mask has a +/-12 nm, or 24 nm total error. The aerial image represents an exposure using 365 nm wavelength exposing light and a projection system with NA=0.50, partial coherence S=0.60 and a reduction ratio of 5:1. As is well known, the dimension of the space in the patterned photoresist layer may be targeted at a dimension different from the reticle dimension by appropriately adjusting the exposure dose. In the present example, it will be assumed that the space is targeted at 300 nm. The curves 101 and 102 represent aerial images of the space for the described situation where the reticle contains the above mentioned small (.+-./-60 nm at the reticle plane for a 5:1 system or .+-.-12 nm at the wafer plane) reticle manufacturing error. That is, curve 101 represents the aerial image of a feature with a -12 nm error (at the wafer plane) and curve 102 represents the aerial image of a feature with a .+-.12 nm error (at the wafer plane). As can be seen from the curves, at the targeted dimension of 300 nm, there is a difference 105 between the aerial image 101 and 102 on each side of the feature. Since the distance 105 on each side is approximately 21 nm, the total difference is approximately 42 nm. Thus, a reticle manufacturing error of approximately 24 nm will result in a dimension variation of approximately 42 nm in the patterned resist layer for a periodic space.
FIG. 2 shows an aerial image of a contact to be targeted at 300 nm (900 nm pitch) with a positive tone mask using the same patterning conditions as in FIG. 1 i.e., 365 nm wavelength exposing light and projection system with NA=0.50, partial coherence S=0.60 and reduction ratio of 5:1. The curves 201 and 202 represent aerial images of the contact for the described situation produced by a reticle containing the same small (.+-./-60 nm at the reticle plane or .+-./-12 nm in the wafer plane) reticle manufacturing error as the space of FIG. 1. As one can see, in the case of a contact, the same reticle manufacturing error of 24 nm will result in a dimension variation 205 of approximately 51 nm per side, or 102 nm total, in the resist, which represents an increase in variability by roughly a factor of 2.5 compared with the space.
The effect of variability in the size of the patterned contact/via opening, due to reticle error and other causes can be seen in FIG. 3. In the graph of FIG. 3, reticle CD refers to the dimension of the opening on the reticle, and actual CD refers to the dimension of the opening in the photosensitive layer after development. (Alternatively, the actual CD may refer to the dimension of a feature in an underlying layer after an etch step and removal of the photosensitive layer.) For a reticle CD 301, which is significantly larger than the minimally resolved feature, and which may have a value of approximately 1.5%/NA, for example, the actual CD remains within the range 301a. At lower reticle CDs, such as reticle CD 102, the range 102a of actual CDs increases due to the above described increased sensitivity to reticle manufacturing errors, as well as higher sensitivity to residual lens aberrations and loss of contrast in out of focus conditions for a smaller feature. For example, design CD 302 may represent a design CD in the range of approximately 1.0.lambda./NA. Finally, for very small reticle CDs, such as design CD 303, which may be in the range of approximately 0.4-0.6.lambda./NA, the range of actual CDs 303a becomes larger still. As can be seen, in some cases the actual CD may have a value of zero, i.e., at least some photosensitive layer remains in all portions of the opening. It will be understood that FIGS. 1-3, and the exemplary dimensions discussed above, are for the purposes of illustration and actual values of reticle CDs and resultant ranges of actual CDs, will vary based upon the technology utilized. However, for any given technology, at some dimension, such as reticle CD 303, there is too great a variation in the size of the opening induced by random and systematic equipment and process variations for the process to be considered manufacturing worthy.
Because the patterning of contact/via openings is far more difficult than the patterning of other features, one prior art approach is to use an opening size approximately 25% or so larger than the minimum line or space dimension which can be achieved in the same technology. For example, while the line and space features may be defined with a reticle CD having a value such as reticle CD 303 of FIG. 1, contact and via openings are defined with a reticle CD having a value 302 of FIG. 1. One drawback with this approach, is that the pattern density is increased compared with openings having the same dimension as the minimum line and space dimension.
Several attempts in the prior art to reduce the variability of small openings have concentrated on providing improved contrast characteristics for contact via patterning in out of focus conditions as described in following paragraphs.
A method which may be used to improve the image contrast in small openings is the focus latitude enhancement exposure (FLEX) method. In this method, multiple exposures of the same reticle at different focal planes is performed. The exposure at each focal plane is partial, such that the total exposure dose is sufficient to image the opening throughout the thickness of the photosensitive layer. In this method, it is difficult to precisely control the focusing and dosage at each focal plane as needed to provide improved control of the feature size variation.
Another approach to produce contact and via openings at minimum geometry is the use of phase-shifted masks. In this method, the central radiation transmitting region of the reticle corresponding to the opening is surrounded by a region which transmits radiation phase-shifted 180.degree. from that transmitted through the central opening portion. The destructive interference which occurs between the opening and the phase-shifter results in improved contrast for a relatively larger range of out of focus conditions. However, dimension tolerance to reticle manufacturing errors is reduced even more due to the dual nature (both amplitude and phase) of reticle errors present. In addition, PSM is a relatively new technology, which still has problems such as mask defects, phase control, need for proximity correction, and others, before becoming a manufacturable technology.
What is needed is a method and apparatus for manufacturably producing openings such as contact and via openings at the minimum feature dimension size for a given technology. The method and apparatus should provide openings with a smaller dimension than currently achievable with the degree of dimensional control that is equal to or better than that for other lithographic features such as lines or spaces. The method and apparatus should allow for improved control over the size variation of the openings of both reduced dimension openings as well as conventionally sized openings. The method and apparatus should provide a robust contact and via lithography process having good ability to control variation in the contact/via size in the presence of sources of variance typical of a given generation of technology. Further, the method and apparatus should not require difficult to implement exposures at several focus planes, should not involve unproven technologies, and should not require expensive and complex equipment beyond what is required for patterning other features.